Make the metaclass of Refinement explicitly

Otherwise, singleton methods of Module are not inherited unless
Refinement.singleton_class is called.

class.c

@@ -689,6 +689,8 @@ Init_class_hierarchy(void)
     RBASIC_SET_CLASS(rb_cObject, rb_cClass);
     RBASIC_SET_CLASS(rb_cRefinement, rb_cClass);
     RBASIC_SET_CLASS(rb_cBasicObject, rb_cClass);
+
+    ENSURE_EIGENCLASS(rb_cRefinement);
 }
 
 

test/ruby/test_refinement.rb

@@ -2668,6 +2668,10 @@ class TestRefinement < Test::Unit::TestCase
     end
   end
 
+  def test_inherit_singleton_methods_of_module
+    assert_equal([], Refinement.used_modules)
+  end
+
   private
 
   def eval_using(mod, s)